I'm notoriously bad at figuring out video games but was able to grok this at the age of 7. It probably had more to do with the fact that education in the 90s placed a decent emphasis on computer literacy (e.g. "Mouse Practice" for Mac Classic) so I was hip to the drag-and-drop paradigm. I don't have kids but I've read that most grow up on touch interfaces these days due to the ubiquity of tablets so I'd imagine that the mouse context is foreign to them.